Make the qt5_use_modules public API.

Make the qt5_use_modules public API.

This cmake function handles all of the necessary logic for using the
include directories of Qt modules, linking to Qt modules, adding
the required definitions, and most importantly, adding the position
independent flags required on UNIX systems to use Qt by default.

The function relies on functionality available in CMake 2.8.8, so it
is only available if that version of CMake or greater is used.

Change-Id: Ibe698e06819129479348c240844264c41553b5fb
Reviewed-by: Alexander Neundorf <neundorf@kde.org>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>